I. Field of the Invention
The invention relates to a method and apparatus for controlling spurious transmissions in a communications system. More specifically, the invention relates to a method and apparatus for controlling spurious transmissions during inter-burst changeovers in a communications system.
II. Description of the Related Art
Modem communication systems, such as cellular and satellite radio systems, employ various modes of operation (analog, digital, dual mode, etc.), and access techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and hybrids of these techniques.
In FDMA systems, a communication channel is a single radio frequency band into which a signal's transmission power is concentrated. Interference with adjacent channels is limited by the use of bandpass filters which only pass signal energy within the specified frequency band. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by radio channels.
In TDMA systems, a channel consists of a time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. A given signal's energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, the portion of the interference from different relative signal strength levels is reduced. In North America, a digital cellular radiotelephone system using TDMA is called the digital advanced mobile phone service (D-AMPS), some of the characteristics of which are specified in the TIA/EIA/IS-136 standard published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA).
Global System for Mobile Communication (GSM) is based on technologies of advanced TDMA. The general packet radio system (GPRS) is a packet data based communication system that has been developed for GSM networks with the aim of providing networks built to this standard with a way to handle higher data speeds and packet switched connections. GPRS can also be used in TDMA networks (IS-136). It is intended to provide a transitional path to third generation (3G) wireless data services. It enables the introduction of packet switching and Internet Protocol (IP). The GPRS standard is now well defined and is currently being deployed in existing GSM-based mobile networks, in order to provide a way for GSM operators to meet the growing demand for wireless packet data services.
An example in the trend toward wireless evolution has been the development of the Enhanced Data Rates for GSM Evolution (EDGE) which is currently under standardization within the European Telecommunication Standards Institute (ETSI). The EDGE specification has been developed so as to offer packet data communications at high speeds within current frequency bands and is based on the current GSM TDMA frame structure, logical channel structure and 200 kHz carrier bandwidth. The current GSM and D-AMPS installed base stations can be upgraded in a step-wise manner for a gradual evolution to the higher bit rates. This enables systems operating in accordance with both network standards to achieve improved bit-rate performance within current frequency allocations and in existing cell plans.
In TDMA systems and even to varying extents in CDMA systems, each radio channel is divided into a series of time slots, each of which contains a burst of information from a user. The time slots are grouped into successive frames that each have a predetermined duration, and successive frames may be grouped into a succession of what are usually called superframes.
In GSM, for example, a single full-rate frame contains 8 time slots with a time duration of 4.615 ms and 1250 bits. Each time slot consists of 156.25 bits with a time duration of 0.57692 ms. GSM uses five different types of the bursts (time slots): normal burst, synchronization burst, frequency correction burst, access burst, and dummy burst.
FIG. 1a shows normal burst structure for GSM. The normal burst is used to carry information on traffic channel and control channels. This burst contains 156.25 bits. It is defined as 3 Tail Bits, 57 Coded Bits, 1 bit Stealing Flag, 26 bit Training Sequence, 1 bit Stealing Flag, 57 Coded Bits, 3 Tail Bits, and 8.25 bits of guard period. The encrypted bits are 57 bits of data or speech plus one bit stealing Flag to indicate whether the burst was stolen for Fast Associated Control Channel signalling or not. The training sequence is a 26 bit pattern which is used by the equalizer to create a channel model. The tail bits are always equal (0,0,0). The guard period 12 is empty space and is used to prevent overlap between adjacent time slots during transmission. They are used to provide start and stop bit patterns. While 1 symbol corresponds to 1 bit for GSM/GPRS, 1 symbol corresponds to 3 bits for EDGE.
FIG. 1b shows radio frequency (RF) Power Amplifier (PA) ramp profile for 1 burst. During guard period corresponding to 8.25 bits, RF PA power control signal is powered down following line 14 and powered up following line 16 for the next burst.
In the mean time, in the higher classes of GSM, GPRS/EDGE and other multi-slot TDMA systems, the Mobile Station (MS) is allowed to transmit more than a single TDMA burst on the UpLink (UL). This is to achieve higher data rates. Where the Mobile Station (MS) is allowed to transmit several adjacent bursts on the UpLink (UL), the transmitting MS has to resynchronise its Transmitter during the guard period between UL bursts.
Since the guard period 12 lasts for 8.25 bits, which means that in a multi-slot situation there is a problem which requires the modulator to be re-synchronised to the start of the next burst due to the 0.25 symbol remainder. Implicit in this is that after modulating 8 symbols, and getting 25% through modulating the 9th symbol, the modulator has to be re-synchronised to start modulating the data for the next burst. This resynchronisation causes discontinuities in the output of the modulator which results in spurious frequency products being sent through the transmit path of the mobile station (MS). Since the RF PA power control remains on, the spurious frequency products are radiated over-air such that they can violate the spectral mask defined for GSM/GPRS/EDGE and contribute to increased system noise.
The problem may be better understood from consideration of the accompanying FIG. 2, which shows an example of prior art where the MS transmits two TDMA bursts on the UL. The second burst here uses higher power level than the first burst. The guard period 22 lasts for 8.25 bits during which the modulator should be re-synchronised to the start of the second burst 2 due to the 0.25 symbol remainder. In FIG. 2, after modulating 8 symbols corresponding to time period 24 and getting 25% through modulating the 9th symbol corresponding to time period 26, the modulator has to be re-synchronised to start modulating the data for the second burst 2 causing discontinuities in the modulator output and imposing spurious frequency products on the second burst 2.
Therefore, when the resynchronisation is left too late and done just prior to the first Tail Bit of the next burst, there is a risk that the Tail Bits may suffer distortion. It will thereby degrade system performance. To avoid that, some receivers will require the Tail Bits to be especially well formed. A similar situation exists with regards to the last Tail bit of the previous burst when the resynchronisation is left too early and done just after the last Tail Bit of the former burst.
One method is to ignore the quarter bit and add a “leap symbol” to every 4th frame, so the guard period for 4 consecutive bursts become 8, 8, 8, 9 bits in length. FIG. 3 shows this scheme. Each of the first three consecutive slots i.e., slots n, n+1, n+2 has 8 bits of guard period ignoring the quarter bit. But the fourth slot i.e., slot n+3 has 8 bits of guard period plus a leap symbol. ETSI spec allows multiple-timeslot user to transmit in this way.
However, this scheme is unsatisfactory because, although it may work in practice, it no longer meets the specification for GSM/GPRS timing. The Network would see a quarter symbol “dither” on the timing of the MS which may reduce the accuracy of the time tracking to half-symbol or worse.
In the meantime, since in EDGE and Dynamic synchronous transfer mode (DTM) operation each burst of multi-slot is independently controlled and the modulation changes, the above alone is not enough. For example, defined by ETSI GSM 05.04, EDGE (EGPRS) uses two modulation schemes, Gaussian Minimum Shift Keying (GMSK) and 8 Phase Shift Keying (8PSK). The two modulation schemes are different in many ways. The most obvious one is that GMSK has constant amplitude, while 8PSK has variable amplitude. GMSK only modulates the phase and keeps the amplitude constant, while 8PSK modulates both phase and amplitude. By doing this 8PSK triples GMSK transmitting data rate.
The introduction of EDGE (and GPRS) means that in the near future a mobile station (MS), i.e. a mobile telephone, will be able to make a voice call at the same time as a data call. That often requires both GMSK and 8PSK modulations in time slot next to each other. This means that both modulation schemes can be in the same spectrum and can appear in two adjacent bursts on both downlink and uplink. For downlink BS transmitter normally does not switch off ramp down at end of each burst, as it needs to transmit in the next burst. For uplink, when there are two adjacent time slots, similar to BS, it is desirable not to power down at the end of the first burst and power up at the beginning of the next one as described in FIG. 2. However, as the power is on in the guard period, it has to be controlled carefully to minimise the interference to others.
The standard specification has defined a spectrum mask for the transition. It needs joint efforts from both baseband and RF to satisfy the requirement and it is desirable to keep the emission as low as possible.
Moreover, if the two bursts are of different modulations, for example, GMSK followed by 8PSK or the other way, the issue arising from such case is that the direct transition between the two modulations often generates spurious spectrum from the output stage of baseband signal, which will appear in RF and cause violation of the mask. Therefore it is necessary condition for both BS and MS baseband to be able to handle transition between adjacent GMSK and 8PSK bursts without generating unwanted frequency components. Since the transition between GMSK and 8PSK may be necessary on top of the power changes, it could cause extra frequency components without further developed control or process.